When the energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when it is exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with the stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer so as to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed by a gas phase-accumulation method or by a firing method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
JP-B-4-75480 discloses a radiation image storage panel which comprises a phosphor layer containing a mixture of two groups of stimulable phosphor particles having the same chemical composition but having different sizes. The group of smaller particles has a size distribution in which the peak is positioned in the range of 1 to 8 μm, and the group of larger particles has a size distribution in which the peak is positioned in the range of 4 to 30 μm. The space between those peaks is at least 2 μm. In the mixture, the ratio by weight between the smaller particles and the larger particles is in the range of 20:80 to 90:10.
JP-A-9-269400 discloses a radiation image storage panel comprising a phosphor layer in which the packing degree of phosphor particles is 65% or more. The phosphor layer contains a mixture of smaller phosphor particles and larger phosphor particles. The smaller particles have a size distribution in which the peak (A) is positioned in the range of 0.5 to 5 μm, and the larger particles have a size distribution in which the peak (B) is positioned in the range of 6 to 30 μm. The ratio by weight between the smaller particles and the larger particles is in the range of 5:95 to 40:60. The particles of the peak B are larger than the particles of the peak A by 5 μm or more, or otherwise are three times or more as large as the particles of the peak A.
JP-A-2004-137359 discloses a process for preparation of a rare earth activated alkaline earth metal fluoride halide stimulable phosphor. The disclosed process comprises a step of classification using a curved pneumatic conveying classifier. In the publication, however, the treated phosphor is in the form of uniform particles.
For the purpose of improving the sensitivity and the image quality, it is known, as disclosed in the above-mentioned publications, that the phosphor layer is made to contain two groups of phosphor particles having different sizes. That is because large and small particles can be densely packed to improve the packing density of phosphor in the phosphor layer. However, if the larger particles are too large, the image quality such as sharpness is lowered and the size distribution is liable to widen out. On the other hand, if the smaller particles are too small, they often agglomerate and the amount of emission decreases to give poor emission properties. Accordingly, it is desired to produce a phosphor layer in which phosphor particles giving preferred properties are so densely packed that the image quality such as graininess can be improved without impairing the sensitivity.